Fuel cells have great potential for supplying inexpensive and clean electrical power. One common type of fuel cell is the hydrogen fuel cell. The basic operation of the hydrogen fuel cell includes the migration of hydrogen ions through a semi-permeable membrane known as an electrolyte membrane (or layer). Another type of fuel cell is the solid oxide fuel cell (SOFC). The SOFC works in part by migrating oxygen ions through the electrolyte layer. For any fuel cell, the ideal electrolyte layer will transport only the desired type of ion.
A fuel cell is an electrochemical device that produces electrical current from chemical reaction. The fundamental device includes an ion-conducting electrolyte between two electrodes, backed by fuel and oxidant flow distributors. At the oxidant side a catalyst on the electrode promotes combination of ions and electrons. At the fuel side a catalyst on the electrode promotes separation of ions and electrons. Only the ions conduct through the electrolyte while the electrons are conducted through an external circuit, thus supplying electrical power. SOFC's have oxygen ion-conducting metal oxide membranes as their electrolyte layer. The oxygen molecules transform into oxygen ions by receiving electrons from electrode/catalyst at the oxidant side. The oxygen ions propagate through the electrolyte membrane and combine with hydrogen molecules into water by leaving electrons to electrode/catalyst at the fuel side. A gas sensor has same basic configuration, and produces electrical current that depends on difference of gas concentration.
Fuel cell operation is increasingly efficient when the electronic conductivity of the electrolyte is minimized and the ionic conductivity of the electrolyte is maximized. It is well known that a fuel cell is thermodynamically more efficient at lower temperatures, with lower entropic losses resulting in a higher open cell voltage.
SOFC's have a several of advantages compared to hydrogen fuel cells including: no humidity requirement for ion exchange, no water clogging up with generated water, no or less noble metal catalyst, high CO tolerance, and useable waste heat.
However, conventional SOFC's also have problems. One of the main problems to be overcome is preparation of hermetic seals at high temperatures. With operating temperatures decreasing from 1000° C. to 700° C. or less, metal materials can be used for sealing and the sealing problem becomes manageable. Many efforts have been made to decrease the operating temperature of SOFC's to below 700° C. despite a large loss of output power. However, these operating temperatures are still too high for mobile applications.
FIG. 1 shows a prior art electrolyte and porous electrode combination. The porous nature of the electrode 104 means that the thickness of the electrolyte 102 is quite large. The porous electrode 104 allows gases to reach the electrolyte 102. The electrolyte 102 should have sufficient thickness such that there are no gaps in the electrolyte 102 as the electrolyte 102 is deposited on the electrode 104. The resulting thick electrolyte 102 layer leads to high resistance.
FIGS. 2A-B show a prior art electrolyte and dense electrode combination. This combination can be seen in U.S. Pat. No. 6,645,656. A dense electrode 204 contacts an electrolyte 102 layer. The electrode 204 is etched, as seen in FIG. 2B, to allow gases to reach the electrolyte 102.
SOFC's have adopted stabilized zirconia for oxygen ion conducting electrolyte layers for several decades. Due to the low ionic conductivity at low temperature, such SOFC's have to be operated above 800° C. High operation temperature limits the choice of materials for stacking and sealing and brings in numerous problems (corrosion and degradation for example). These problems have so far resulted in high costs and limited applications, even though SOFC's have many advantages over the other power systems (environment protection for example). Therefore, lowering the operating temperature of a SOFC in a stationary power system is desirable. Other potential applications, including electric vehicles and portable electronics, are another driving force to lower the operating temperature of SOFC. One way to achieve lower operating temperatures is by choosing ceramic electrolytes with higher oxygen ion conductivities at lower temperature. Another way is by reducing the thickness of the electrolyte membrane.
Doped ceria is one of the suitable electrolyte candidates for low-temperature SOFC. One common dopant is Gd2O3 and typical composition for Gd-doped Ceria is Gd0.2Ce0.8O1.9-x (GDC). Oxygen ion conductivity in doped ceria is understood to be two to three orders higher than that of yttria stabilized zirconia (YSZ) at low temperatures. Doped ceria has not been successfully used in a SOFC because it will transfer into a mixed conductor under reduced atmosphere and as a result short-circuit the cell at around 700° C. Fortunately, the ionic domain of doped ceria increases as the temperature decreases. At a temperature of 500° C., with a favorable SOFC anodic condition, the ionic transference number of doped ceria is larger than 0.9. Therefore, doped ceria is one of the suitable candidates for low temperature SOFC.
Thus, there is a need for a solid oxide electrolyte membrane with high ionic conductivity and physical properties that allow solid-state ionic devices to be able to operate at low temperatures.